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Escherichia Coli: trp operon
Introduction In E.coli, Tryptophan operon consists of five structural genes (trpE, trpD, trpC, trpB, &trpA) that encode enzymes needed for the biosynthesis of amino acid tryptophan. The five genes are all transcribed together as a unit known as operon and is controlled by an operon. Bind of a repressor (regulatory protein) to the operator site to prevent the transcription. Two other genes involved in trp operon regulation are the trpR, which encodes repressor protein and trpL, discovered Yanofsky and coworkers, is involved in attenuation. Two separate pathways, repressible system and attenuation, can regulate the trp operon. At low level or concentration of tryptophan, the repressor is unable to bind to the operator site and cannot inhibit transcription. Under this condition the RNA polymerase can bind to the promoter site and transcribe the operon genes required for tryptophan biosynthesis enzymes. When level of tryptophan is high, the trp repressor binds to the trp operator and activates it, thus the synthesis of tryptophan is shut down or blocked. Yanofsky and colleagues sequenced and isolated mutant strains that lack the trp repressor protein (leader sequence or trpL). To their surprise, the mutant strain, approximately 160 bases at the 5’ end is able to inhibit the expression of trp operon in the presence of tryptophan. Yanosfsky called the element inactivated by the deletion the attenuator, as a result of reduction in the rate of mRNA transcription. The regulatory mechanism of the trpL is termed attenuation. Attenuation of the trp operon involved structure and sequences of the trpL mRNA. Interactions among several complementary regions (region1, 2, 3&4) of the trpL mRNA can form stem loop structures in the RNA transcript. The four regions are numbered relative to the 5’-end of the transcript. The regions are capable of forming hydrogen bonding between region 1and 2, 2 and 3 and 3 and 4. Region 2 is both complementary to 1 and 3, thus can form stem loop structure with both 1 and 3 separately. Also, region 3 is complementary to regions 2 and 4. The formation of stem loop between regions 1 and 2, leads to formation of stem loop between 3 and 4 regions. Following region 4 is the last uracil (U)-rich sequence termed attenuator. When region 3 and 4 stem loop is present, it acts with the attenuator to form a terminator for transcription at the end of the trpL gene. Furthermore, when region 2 and 3 stem loop is present, the terminator structure does not form and transcription proceeds to the trpE and other tryptophan biosynthesis genes in the operon. In absence of translation, the trpL mRNA is in it most stable conformation. Trp regulation References Bennett G., Schweingruber M., Brown K., Squires C., and Yanofsky C. Nucleotide sequence of the promoter-operator region of the tryptophan operon of Escherichia coli. J Mol Biol 1978 (121) 113-137.] Griffiths AJF, Miller JH, Suzuki DT, et al. An Introduction to Genetic Analysis. 7th edition. New York: W. H. Freeman; 2000. Additional examples of control: attenuation.